Liquid crystal display devices have been applied to, for example, watches, calculators, a variety of household electrical appliances, measuring equipment, panels used in automobiles, word processors, electronic notebooks, printers, computers, and television sets. Representative examples of types of liquid crystal display devices include a TN (twisted nematic) type, an STN (super twisted nematic) type, a DS (dynamic scattering) type, a GH (guest-host) type, an IPS (in-plane switching) type, an OCB (optically compensated birefringence) type, an ECB (electrically controlled birefringence) type, a VA (vertical alignment) type, a CSH (color super homeotropic) type, and an FLC (ferroelectric liquid crystal) type. Regarding a drive system, multiplex driving has become popular instead of typical static driving; a passive matrix, in particular, an active matrix (AM) in which, for example, a TFT (thin film transistor) or a TFD (thin film diode) is used for driving has become mainstream in recent years.
In a method for manufacturing liquid crystal display devices, a dropping technique involving use of an optically and thermally curable sealant is widely used. In the dropping technique, a rectangular sealing pattern is formed on one of two transparent substrates each having an electrode by using a dispenser or by screen printing. Then, fine droplets of liquid crystal are dropped onto the entire surface of the transparent substrate inside the frame in a state in which the sealant has not been cured yet, this transparent substrate is immediately attached to the other transparent substrate, and then the sealing part is irradiated with ultraviolet to be temporarily cured. Then, the sealant is completely cured by heating in a process of liquid crystal annealing to produce a liquid crystal display device. Attaching the substrates to each other under reduced pressure enables production of liquid crystal display devices with significantly high efficiency.
Since the dropping technique has a process in which an uncured sealant directly contacts liquid crystal material, the liquid crystal material is contaminated by a component of the sealant, which has been greatly problematic. In addition, residues, such as an unreacted polymerization initiator and a curing agent, and ionic impurities contained in a cured sealant have been also problematic. These days, in liquid crystal panels, liquid crystal driven at low voltage (low-voltage liquid crystal) tends to be used for the purpose of a reduction in power consumption, for instance, in application to mobile devices. Such low-voltage liquid crystal has a particularly large dielectric anisotropy and therefore easily takes in impurities, which readily results in defective alignment and a reduction in a voltage holding ratio over time. In particular, residues, such as an unreacted polymerization initiator and an initiator after curing, ionic impurities such as chlorine, or silane coupling agents contained in a sealant flow into a liquid crystal material, and thus problems such as defective alignment and a reduction in a voltage holding ratio over time are caused.
In such circumstances, in order to prevent a component of a sealant from flowing into a liquid crystal material, there has been a suggestion in which the softening point of an epoxy resin contained in the sealant is enhanced for prevention of contamination of the liquid crystal material due to contact thereof with an uncured sealant and for a reduction in color unevenness (Patent Literature 1). Another suggestion for preventing a component of a sealant from flowing has been made; the sealant having a composition which enables both optical curing and thermal curing is prepared, the sealant is applied and then temporarily cured by being irradiated with light in order to avoid contamination brought about by contact thereof with a liquid crystal material, two substrates are attached to each other, and then the sealant is completely cured by heating (Patent Literature 2). In order to enable this suggestion, an acrylic-acid-modified epoxy resin produced by the reaction of an epoxy resin with acrylic acid is used as a component of the sealant.
In general, epoxy resins have a high adhesiveness but greatly tend to contaminate liquid crystal materials. It is expected that the above-mentioned modification with acrylic acid also contributes to a reduction in contamination of liquid crystal materials. The modification with acrylic acid, however, impairs thermosetting properties, which results in contamination of a liquid crystal material due to flowing of a component of the sealant thereinto in some cases. Another suggestion has been therefore made in order to cure an acrylic component; a tertiary amine such as imidazole is added to thermally cure an acrylic resin owing to the interaction thereof with a small amount of an epoxy resin which has been also added (Patent Literature 3).
In each of the suggestions, however, liquid crystal materials generally used are considered, attention is paid to the composition of the sealant, and a change is made to the composition of the sealant with the aim of solving the problems. Hence, in the case where such suggestions are applied to individual liquid crystal display devices, the liquid crystal display devices do not necessarily have sufficient display properties in many cases; in particular, the screen burn-in of a liquid crystal display device has not been sufficiently overcome.